The present invention relates to a method and a device for detecting an output current of an inverter by calculation therefor.
FIG. 2 shows an example of an output circuit in a conventional inverter. Generally, an output of an inverter circuit used in an uninterruptible power supply is filtered by an LC filter made up of a reactor L and a capacitor C before being output. A control of the inverter requires a detection of an output current iL of the inverter circuit. For this purpose a current detector is provided at a position 101 in FIG. 2. When there is a need to detect an output current (load current) io of the inverter, a current detector is provided at a position 102 in FIG. 2.
When both of the output current iL of the inverter circuit and the load current io of the inverter need to be detected, two detectors are required. For a reduction of size and cost, some inverters use the output current iL of the inverter circuit rather than detecting the output current io. It is noted, however, that since a capacitor current ic flows through the capacitor C, io is not equal to iL. Because io=iL−ic, there is an error. If io>>ic, the effect of this error on the phase is small. But as the magnitude of the capacitor current ic becomes closer to that of the output current io, not only the magnitude of this error but also the adverse effect of the error increase.
According to a general theory, the capacitor current ic can be obtained according to the expression (1) as follows.ic=C×dvo/dt  (1)where C is a capacitance of the capacitor and vo is an output voltage (=voltage across the capacitor). As can be seen from this expression, the capacitor current ic can be obtained by differentiating the output voltage vo. Since the output voltage vo is a sinusoidal wave and differentiated to calculate the capacitor current ic, it is seen that the capacitor current ic leads the output voltage vo by 90 degrees in phase. By calculating the expression (1) using a CPU (central processing unit) or a DSP (digital signal processor), the capacitor current ic can be obtained from the output voltage vo. However, differentiating the output voltage vo using a low-performance CPU or DSP is technically difficult.
An object of this invention is to provide a method for detecting inverter output current and a device capable of calculating the output current of an inverter easily using a low-performance calculating device without lowering a calculation accuracy.